The present invention relates to a horizontal synchronous signal separation circuit used with video apparatus such as video tape recorder, television receiver, projector and the like, and more particularly to an improvement on detection timings of a synchronous signal.
An example of a conventional horizontal synchronous circuit is shown in FIG. 1. In FIG. 1, a video signal input terminal 10 is connected to the input side of a synchronous tip clamp circuit 12 whose output side is connected to the negative input of a comparator 14. A reference voltage VR1 is inputted to the positive input of the comparator 14 to compare an input to the negative terminal with the reference voltage VR1. The output side of the comparator 14 is connected to an output terminal 16.
The operation of such a conventional circuit will be described with reference to FIG. 2. A video signal inputted to the terminal 10 is subjected to DC restoration at the synchronous tip clamp circuit, and thereafter compared with the reference voltage VR1 by the comparator 14 (refer to FIG. 2(a)). A composite synchronous signal shown in FIG. 2(b) is thereby outputted to the terminal 16.
Since a conventional circuit separates a horizontal synchronous signal by comparing the dc-restored video signal with the reference voltage, the obtained composite synchronous signal is in synchronism with the leading edge of the synchronous signal within the inputted video signal. In the synchronizing system of a video apparatus such as a video tape recorder, video signals are processed by using as the synchronous timing reference the leading edge of the composite synchronous signal.
There is however associated with some problem that the timing of the leading edge of a synchronous signal contained in a video signal reproduced from a video tape recorder for example is likely to be affected by the contents of the video signal, resulting in an erroneous detection of the synchronous signal.
More specifically, as shown in FIG. 3, an original video signal S1 shown in FIG. 3(a) recorded and reproduced by a video tape recorder shown in FIG. 3(b) may sometimes become a signal S2 as shown in FIG. 3(c). In such a case, a conventional circuit will separate a composite synchronous signal S3 as shown in FIG. 3(c). Therefore, the leading edge of the separated synchronous signal has a shift t from the leading edge of the synchronous signal contained in the original video signal S1. Noises are therefore generated in the synchronizing system of video apparatus, resulting in an unstable operation of the synchronizing system.